System for simultaneously scrubbing cement kiln exhaust gas and producing useful by-products therefrom

ABSTRACT

This invention provides a system for simultaneously scrubbing acidic exhaust gases and removing alkali and alkaline earth metal salts from waste dust from a cement plant or the like thereby producing useful products from what would otherwise be wasted. In essence the two wastes are combined in water and the resulting solution and precipitate separated in a tank. The solution is useful both to cool incoming waste gas and as a fertilizer, while the precipitate is suitable as kiln feed stock.

This application is a division of application Ser. No. 904,719, filedSept. 5, 1986, entitled METHOD AND APPARATUS FOR SIMULTANEOUSLYSCRUBBING CEMENT KILN EXHAUST GAS AND PRODUCING USEFUL BY-PRODUCTSTHEREFROM now U.S. Pat. No. 4,716,027 which, in turn, is acontinuation-in-part of application Ser. No. 890,991, filed July 25,1986 now U.S. Pat. No. 4,708,855.

This invention relates to the treatment of kiln dust and exhaust fumesfrom a cement facility to convert the former into a useful product whileremoving pollutants from the latter.

BACKGROUND OF THE INVENTION

Dust collection facilities operating at cement or lime producing kilnsremove particulate matter from the kiln exhaust. This particulate matterincludes calcium carbonate, calcium oxide, and the oxides and carbonatesof other metals, depending on the composition of the feed stock to thekiln. Two elements frequently present in the kiln dust are potassium andsodium. These elements limit or prevent the reuse of the kiln dust askiln feed stock, since they interfere with the properties of the finalproduct, and the dust is therefore discarded. These dust collectionfacilities do not remove sufficient gaseous pollutants from the exhauststream and separate scrubbers must be provided if they are to beprevented from entering the atmosphere.

The use of alkali or alkaline earth materials, as solids or in a slurryor in a solution, for scrubbing (removing sulfur and nitrogen oxides) ofexhaust gas has been known for years (see discussion in Morrison, G.,supra, hereby incorporated by reference). For example, Mehlmann (1985,Zement-Kalk-Gips Edition B) describes the use of hydrated or pulverizedlimestone at temperatures up to 1100° C., or of spray drying withhydrated lime; and Ayer (1979, EPA-600/7-79-167b) describes the use oflime to scrub exhaust gas from a heating plant. Limestone may beincluded in the charge of fluidized bed furnaces for the same purpose.In general the oxides of carbon, sulfur, and nitrogen, present inexhaust gas, when reacted with water, produce acids, including sulfuric,sulfurous, nitric, and carbonic. The presence and amounts of each dependon the oxides present, the availability of oxygen, and the reactionconditions. When these acids are reacted with the oxides, hydroxides, orcarbonates of alkali or alkaline earth metals, salts of the componentsare formed. For example, sulfuric acid will react with the calciumcarbonate present in limestone to yield calcium sulfate.

The principal object of the present invention is to provide a system foreliminating the dust disposal problem present in cement plants andsimultaneously reducing gaseous and particulate emissions and to be ableto do so cost effectively, including the conversion of otherwise wasteproducts into useful products.

BRIEF SUMMARY OF THE INVENTION

This invention provides a system for removing a significant portion ofthe potassium, sodium and sulfur in kiln dust, so that the dust may bereused as kiln feed; and at the same time uses the kiln dust as areactant for removing the oxides of sulfur, nitrogen and carbon from theexhaust stream.

More specifically, by means of the system, cement kiln dust or lime kilndust is reacted with acid produced from the acidic oxides of sulfur,nitrogen and carbon in exhaust gas to dissolve some components of thedust and render the remaining undissolved solids reusable as processfeed stock; a portion of the oxides of sulfur, nitrogen and carbon isscrubbed from the exhaust stream and made available for reaction withkiln dust; as a by-product alkali and alkaline earth metal saltscomposed of the scrubbed exhaust gas sulfur, nitrogen and carbon oxidesand materials derived from the kiln dust are produced; and heat in theexhaust stream is employed to reduce the water content of the processstream containing the dissolved alkali and alkaline earth metal salts.

In accordance with the invention, there is provide a system forsimultaneously scrubbing an exhaust gas stream having acidic oxides andtreating dust having alkali and alkaline earth metal compounds assolids, comprising (a) means for mixing the dust with water to form analkaline (pH greater than 7) solution and (b) means for contacting thegas stream with the said alkaline solution, whereby the said solutionand gas react to form an alkali and alkaline earth metal salt solutionand a precipitate of alkali and alkaline earth metal salts withinsoluble silicates, aluminates and iron compounds.

In preferred embodiments, means are provided for: (a) cooling and/ordehumidifying the gas stream prior to passing the same into the alkalinesolution; (b) utilizing the alkali and alkaline earth metal saltsolution for the purpose of such cooling and/or dehumidifying; (c)utilizing said precipitate together with insoluble silicates, aluminatesand iron compounds to add to the feed stock of a cement plant to producecement; (d) utilizing the heat of the gas to remove water from thealkali and alkaline earth metal salt solution aforesaid; (e) utilizingthe dissolved solids contents of the alkali and alkaline earth metalsalt solution as a fertilizer; and (f) obtaining the exhaust gas streamand dust from the kiln of a cement plant

In accordance with one aspect of the invention, there is provided asystem for converting dust from a cement kiln containing alkali andalkaline earth metal compounds as solids into a product suitable foradding to the feed stock for said kiln, comprising (a) means fordissolving the soluble portion of the dust in water to form an alkalinesolution, and (b) means for passing exhaust gas from the kiln into thesaid solution, whereby the solution and gas react to form an alkali andalkaline earth metal salt solution and a precipitate of alkali andalkaline earth metal salts with insoluble silicates, aluminates and ironcompounds suitable for adding to the feed stock for the kiln.Preferably, means are provided for cooling and dehumidifying the gasprior to passing it into the alkaline solution, utilizing the latter forsuch purpose, and means are also provided for evaporating water from thealkali salt solution by heat from the gas.

In accordance with a further aspect of the invention, there is provideda system for simultaneously converting dust from a cement kilncontaining alkali and alkaline earth metal compounds as solids into aproduct suitable for adding to the feed stock for the kiln, scrubbingthe exhaust gas stream from the kiln and producing alkali and alkalineearth metal salts suitable for use as fertilizer, comprising (a) meansfor dissolving the dust in water to form an alkaline solution, and (b)means for passing exhaust gas from said kiln into the said solution,whereby said solution and gas react to scrub the latter and at the sametime form an alkali and alkaline earth metal salt solution and aprecipitate of alkali and alkaline earth metal salts with insolublesilicates, aluminates and iron compounds suitable for adding to the feedstock for said kiln, and, lastly, means for evaporating water from thealkali and alkaline earth metal salt solution to produce a concentratedsolution or a precipitate suitable for use as fertilizer.

The system of the invention includes apparatus for simultaneouslyscrubbing a gas stream containing acidic oxides and treating dust from acement kiln or the like containing alkali and alkaline earth metalsalts, comprising a treatment tank, means for feeding a mixture of waterand dust into the latter, means for feeding additional water into thesaid tank further to dilute the mixture aforesaid, means for passing thegas into the mixture to react therewith and thereby to produce anaqueous solution of its water soluble components to form therewith aslurry of acidic oxides and alkali and alkaline earth metal salts andany remaining insoluble compounds, a precipitation tank, means forconveying the slurry from the treatment tank to the precipitation tankwherein the slurry is separated into solids and liquid, the formerdropping toward the base of the tank beneath the latter which remains inthe upper part thereof, means for removing the said solids from the baseof the precipitation tank and means for removing the said liquid fromthe upper part of the precipitation tank.

In a presently preferred embodiment, the apparatus further comprises aheat exchanger, means for passing the said gas through the heatexchanger prior to being fed into the treatment tank and means forpassing liquid removed from the precipitation tank through the heatexchanger, whereby the incoming gas stream is cooled and the said liquidsimultaneously heated to remove the water therefrom.

Still further objects, features and advantages of the invention will beapparent from the following detailed description of a presentlypreferred embodiment thereof taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a schematic representation of one apparatus for practicing theinvention;

FIG. 2 is a graph illustrating the effectiveness of removal of potassiumand sulfur oxides from kiln dust during reaction with exhaust gas oxidesaccording to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The invention comprises means for supplying alkali and alkaline earthmetal compounds as solids, the kiln dust, water, and exhaust gas, andapparatus for first combining and reacting the same and then separatingthe resulting components.

Briefly, the acidic gas and alkaline solids are mixed with water in atreatment tank and react to form a slurry. A settling tank is providedfor the separation of precipitated and undissolved solids from thesolution of water and dissolved solids in the slurry. A heatexchanger-crystallization unit is also provided and utilizes the heat,including latent heat, in the exhaust stream to evaporate water from thesolution of dissolved salts in the slurry to provide cooling of theexhaust gas for use in the above process. Heat for evaporation of waterfrom the solution of dissolved salts is also obtained from the hot wastedust, the hydration reaction between dust and water, and energy obtainedfrom compression of the exhaust gas.

Referring to FIG. 1, a slurry, exiting at 8 from a dust and water mixingtank (not shown), composed of alkali and alkaline earth metal saltsolids and water, is pumped via pipe 10 into the treatment tank 12,along with additional water through inlet 14 from a suitable source (notshown) to produce a dilute slurry 16. Exhaust gas from a cement kiln orboiler (not shown) enters heat exchanger 22 through inlet 18 from whichit emerges as cooled exhaust. Condensed exhaust gas moisture iscollected in the heat exchanger 22 and conveyed to the treatment tank 12through pipe 44. The exhaust then travels to compressor 20 through pipe23 and is delivered via pipe 24 to distribution pipes 26 in the bottomof the treatment tank 12. To prevent settling of the solids to thebottom of treatment tank 12 the slurry may be stirred or recirculated bysuitable means, for example by recirculation pump 27.

The exhaust gas bubbles through slurry 16 of alkaline solids and waterto emerge from the tank top as scrubbed exhaust 28. Slurry 16, as amixture of treated solid, water, and dissolved materials, is pumped bypump 32 via pipe 30 to the settling tank 34 where the settled solids 36are pumped out by pump 38 and the water 37 laden with dissolved salts ispumped to heat exchanger 22 to provide cooling for the input exhaustgas. The water from salt solution 37 is evaporated to a vapor andreleased via pipe 40 to the atmosphere or the water is evaporated andthen condensed to a liquid to recapture the latent heat for reuse. Thesalts from the salt solution 37 are concentrated and/or precipitated andcollected from the heat exchanger via pipe 42. The cationic componentsof the collected salts are principally calcium, potassium, magnesium,and sodium. The anionic components of the salts are principally sulfate,carbonate and nitrate. The actual composition of the salts will dependon the initial composition of the kiln dust to be treated and on thecomposition of the exhaust gas.

The heat exchanger 22 is a dual purpose heat exchanger-crystallizationunit of a known type which will extract heat from the exhaust gas anduse that heat, including latent heat derived from condensation of theexhaust gas moisture to evaporate water.

THE APPARATUS

The whole system is created from well known parts combined by standardmethods. For example, typically the treatment tank may have a volume ofone million gallons and be provided with gas distribution and stirringmeans; the settling tank may have a volume of 100 thousand gallons, bothbeing constructed from stainless steel, or other suitable materials,such as rubber, which can tolerate highly alkaline or acidic solutions.

Working Principle

The basic working principle in this invention is recombination andreaction of two waste streams produced during burning to provide mutualneutralization of the waste streams, production of a valuable and usefulby-product, and utilization of low grade heat energy. The two wastestreams are the gaseous oxides which produce acidic solutions in waterand the waste particulate matter from cement kilns, which produce basicsolutions in water.

After partial dissolution in water, the two wastes react to neutralizeeach other. In the case of cement kiln dust, which contains excesspotassium and/or sodium and sulfate, the process provides fordissolution of a significant proportion of the remaining undissolvedsolids which include calcium and magnesium salts. The resulting solidsare therefore suitable for use as process feed stock. The potassiumsulfate and other salts removed from the heat exchanger-crystallizationunit are suitable as fertilizer or as a source of material forextraction of chemicals. At the same time, the exhaust gas passingthrough the slurry in the treatment tank is cleansed of a significantportion of the oxides of sulfur and nitrogen, by forming sulfates andnitrates.

Example

The discussion that follows is an example of use of the invention in amoderate size wet process cement producing facility.

Exhaust gas from the kiln baghouse, fed through duct 18, is introducedto heat exchanger 22 at a rate of 200,000 actual cubic feet per minuteby compressor 20. The exhaust gas is variable in composition, butcontains roughly 29% water, 25% carbon dioxide, 36% nitrogen, 10% oxygenand 400 to 600 ppm nitrogen oxides and 200 ppm sulfur dioxide. In heatexchanger 22 the exhaust gas is cooled and water is condensed, resultingin a 35% to 40% decrease in flow volume. The exhaust gas is then drawnby compressor 20 through pipe 23 for delivery through pipe 24 todistribution pipes 26 and allowed to react with slurry 16 where most ofthe oxides of sulfur and nitrogen are removed. In lab scale trials 99%of the SO₂ was removed from the exhaust stream.

Kiln dust is introduced to treatment tank 12 at a rate of eight totwelve tons per hour dry weight. Water is added to produce a diluteslurry of up to 95% water content. The slurry water content isdetermined by the initial concentration of potassium and sodium in thewaste dust and on the desired concentration in the material to bereturned to the kiln feed system. After reaction with the exhaust gas,the slurry of treated dust is pumped at a rate of approximately 200gallons per minute to settling tank 34. In this tank the solids settleto form a slurry of approximately 35% water and 65% solids, beneath asolution of water and soluble salts dissolved during treatment. Theslurry is pumped by pump 38 from tank 34 and combined with process feedfor a cement plant at a rate of approximately 7.8 tons per hour ofsolids. The water solution is pumped through outlet 37 to heat exchanger22 at approximately 200 gallons per minute to provide cooling for theexhaust gas and to evaporate the water therefrom to produce theby-product salts. The by-product salts, removed via pipe 42, areproduced at a rate of approximately eight to twelve tons per day. Theby-product salts include potassium sulfate, calcium carbonate, and othersalts with cationic components incuding potassium, calcium, magnesium,and sodium and anionic components including carbonate, sulfate, andnitrate. A portion of the nitrate oxidizes the sulfite to sulfate.

Referring to FIG. 2, the results of two experiments (KD-18 and KD-20 )are displayed, showing that extraction of the alkali and alkaline earthmetal salts from the dust results in the treated dust being acceptableas kiln feed stock. That is, the level of potassium salts falls fromapproximately 3% to less than 11/2%, and the level of sulfate falls fromapproximately 6% to 3% or below. It should be noted that in the examplesillustrated in FIG. 2 a full charge of dust was loaded initially intothe treatment tank and then the introduction of gas commenced. Thisaccounts for the slopes of the graph during days A through M. Thisreduction in potassium, sodium, and sulfate concentration in the dust,from initial untreated to final treated material is greater than 50%.Samples A-M refer to successive days during which samples were takenfrom a continuous treatment process.

The impact of addition of treated dust on raw feed composition is shownin the following table showing the percentage of each oxide in normalkiln feed for both Type I and Type II cement production. The figurescontained in the columns labeled "100 TPD Dust Added to Feed" and "200TPD Dust added to Feed" show dramatically the slight impact oncomposition of the feed resulting from the addition of 100 tons per dayand 200 tons per day respectively, of treated dust to the normal feed.

                  TABLE                                                           ______________________________________                                        Impact of Treated Dust on Kiln Feed Composition                               Normal         100 TPD Dust                                                                              200 TPD Dust                                       Feed           Added to Feed                                                                             Added to Feed                                      ______________________________________                                        Type I                                                                        SiO.sub.2                                                                             12.99      12.99       12.99                                          Al.sub.2 O.sub.3                                                                      3.57       3.59        3.61                                           Fe.sub.2 O.sub.3                                                                      1.45       1.53        1.61                                           CaO     43.49      43.62       43.75                                          MgO     2.83       2.81        2.78                                           SO.sub.3 *                                                                            0.18       0.23        0.28                                           K.sub.2 O                                                                             0.93       0.94        0.96                                           Loss    35.83      35.45       35.07                                          Si Ratio                                                                              2.58       2.54        2.49                                           Al/Fe   2.46       2.35        3.24                                           Type II                                                                       SiO.sub.2                                                                             13.24      13.23       13.22                                          Al.sub.2 O.sub.3                                                                      3.33       3.35        3.38                                           Fe.sub.2 O.sub.3                                                                      1.77       2.03        2.09                                           CaO     43.09      43.23       43.38                                          MgO     2.66       2.64        2.62                                           SO.sub.3 *                                                                            0.19       0.24        0.29                                           K.sub.2 O                                                                             0.68       0.70        0.72                                           Loss    35.20      34.85       34.49                                          Si Ratio                                                                              2.49       2.46        2.42                                           Al/Fe   1.69       1.65        1.62                                           ______________________________________                                         *Sulfate expressed as SO.sub.3                                           

These results demonstrate that the major change in kiln dust compositionis the removal of SO₃ and K₂ O and that the unremoved K₂ O and SO₃ donot significantly alter the composition of the raw feed.

While there has herein been disclosed and described a presentlypreferred system for practicing the invention, it will nevertheless beunderstood that the same is by way of illustration and not by way oflimitation, and it is intended that the scope of the invention belimited only by the proper interpretation to be afforded the appendedclaims.

I claim:
 1. A system for reacting cement kiln exhaust gas containing aspollutants at least one of the acidic oxides of sulfur, nitrogen, carbonand halogen compounds with waste cement kiln dust containing alkali andalkaline earth metal salts utilizing heat from the exhaust gas torecover the reaction products while eliminating waste materials, whichcomprisesa heat exchanger-crystallization unit, a gas-liquid contactingtreatment tank, a settling tank, and means interconnecting said unit andtanks, said interconnecting means comprising conduit means forconducting the exhaust gas through said unit into said gas-liquidtreatment tank and venting after treatment, compressor means for movingthe gas, means for introducing a mixture of water and cement dust tosaid treatment tank for reaction with the gas, means for removing thereacted liquid and solid contents of said treatment tank to saidsettling tank wherein the contents are separated into (a) solidssuitable for use as feed stock for the kiln and (b) a liquid containingdissolved salts comprising potassium sulfate and sodium sulfate, andconduit means for conveying the last named liquid to said heatexchanger-crystallization unit which serves to cool the exhaust gaswhile heating the liquid to the point of evaporation therebycrystallizing the salts contained therein for subsequent recovery. 2.The system according to claim 1 further including means for collectingwater condensed from the exhaust gas in said heatexchanger-crystallization unit for incorporation into the mixture ofwater and cement dust in said treatment tank.
 3. The system according toclaim 1 further comprising means for agitating the mixture of water andcement dust in said treatment tank.
 4. The system according to claim 3wherein said agitating means comprisesa pump and conduit means placingsaid pump in communication with said treatment tank for recirculatingsaid mixture out from and back into said treatment tank.